Data-holding circuit and substrate for a display device

ABSTRACT

A display device has a data-holding circuit with a capacitance and a display portion with a plurality of pixel electrodes, formed on a first carrier substrate. In the display device, a second carrier substrate disposed opposite the first carrier substrate is placed above the display portion, but the opposing substrate is not present above the area in which the data-holding circuit is disposed. The parasitic capacitance of the data-holding circuit can thereby be reduced. Therefore, the capacitance in the data-holding circuit can be reduced and the area required can be reduced as well. The display data of all the pixels is sent serially to the liquid crystal module without high-speed transfer for each frame time interval, and size can be reduced because the controller IC and interface circuit are formed on the same substrate as the display device substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/437,929, filed May 19, 2006, published as U.S. Publication No.2007-0115229, which claims foreign priority of Japanese PatentApplication No. 2005-336426, filed Nov. 21, 2005. The entire contents ofeach are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device used in projectors,laptop PCs, monitors, mobile phones, PDAs, and the like, and to anapparatus that uses this display device.

2. Description of the Related Art

Conventionally, drive circuits and various other circuits in displaydevices are configured with an LSI or the like that is made by usingsilicon technology, and are disposed outside of the display device.However, as technology has developed in recent times, the drive circuitsand various other circuits have come to be mounted on a carriersubstrate of the display devices, and display devices mounted with thesecircuits are being brought into practical use. A known example of adisplay device with such a circuit internally mounted is a displaydevice in which the circuits are configured with a high-temperaturepolysilicon TFT (Thin Film Transistor) that is formed by ahigh-temperature process in which a high-cost quartz substrate is usedas the carrier substrate. Also being placed in practical use are displaydevices in which circuits are mounted on a glass substrate or the likeby using low-temperature polysilicon technology, whereby a precursorfilm is formed with a low-temperature process, and the precursor film isannealed using a laser or the like to produce a polycrystallinestructure.

A specific example is the active matrix display device disclosed inFIGS. 37 and 38 of Japanese Laid-Open Patent Application No.2004-046054. FIG. 1 is a block diagram showing the structure of thedisplay system of the conventional common drive circuit-integratedliquid crystal display device described in FIG. 37 of Japanese Laid-OpenPatent Application No. 2004-046054.

Referring to FIG. 1, integrally formed by polysilicon TFT on a displaydevice substrate 101 in a conventional drive circuit-integrated liquidcrystal display device are an active matrix display area 110 in which Mrows and N columns of pixels are disposed and wired in the form of amatrix, a row scanning circuit (scan line (gate line) drive circuit)109, a column scanning circuit (data line drive circuit) 3504, an analogswitch 3505, a level shifter 3503, and other components.

Also, mounted outside the display device substrate 101 is an integratedcircuit chip (IC chip) in which a controller 113, memory 111, digitalanalog converter circuit (DAC circuit) 3502, scanning circuit/dataregister 3501, and other components are formed on a single-crystalsilicon wafer as a controller IC (Integrated Circuit) 102. An interfacecircuit 114 is formed on a system-side circuit substrate 104 and isconnected to the controller 113 and memory 111.

Also present in a conventional drive circuit-integrated liquid crystaldisplay device structured with polysilicon TFT is a device integrallyformed with a circuit that is more complicated than a DAC circuit andthe like. FIG. 2 is a block diagram showing the structure of the displaysystem of a conventional drive circuit-integrated liquid crystal displaydevice with an internally mounted DAC circuit described in FIG. 38 ofJapanese Laid-Open Patent Application No. 2004-046054.

In a conventional drive circuit-integrated liquid crystal display devicewith an internally mounted DAC circuit, in the same manner as the devicein FIG. 1 that does not have an internally mounted DAC circuit,integrally formed on the display device substrate 101 are an activematrix display area 110 in which M rows and N columns of pixels aredisposed and wired in the form of a matrix, a row scanning circuit 109,and a column scanning circuit 3504, and additionally integrally formedare a data register 3507, a latch circuit 105, a DAC circuit 106, aselector circuit 107, a level shifter (D bit) 108, and other components.

A controller IC 103 that is mounted outside of the display devicesubstrate 101 of the drive circuit-integrated liquid crystal displaydevice with an internally mounted DAC circuit does not include ahigh-voltage DAC circuit 3502, and it is possible to configure thememory 111, output buffer circuit 112, and controller 113 all withlow-voltage circuits and elements. As a result, a controller IC 103 canbe fabricated without the simultaneous use of high-voltage processesthat require voltage signals to be generated for writing to the liquidcrystal. Therefore, the cost of the controller can be made lower thanthat of the controller IC 102 in which the DAC circuit 3502 describedabove is also mounted.

However, the drive circuit-integrated liquid crystal display devices ofthe prior art transfer display data of all of the pixels to the liquidcrystal module serially at high speed for each frame time interval.Therefore, as a result of higher definition, the required transfer ratecommensurate with the increase in the number of pixels becomes muchhigher. And for high speed transfer, the driver IC also must operate athigher speed, a through-current or the like is generated in the largenumber of CMOS (Complementary Metal Oxide Semiconductor) componentsconstituting the circuit elements, and power consumption increasestogether with the increase in operating speed. ICs that operate at highspeed also have higher cost. The complexity and transfer speed of thecircuit structure then increases when the gradation increases, leadingto further increases in power consumption and higher costs. Morespecifically, since the price and power consumption of the driver ICincrease together with higher definition and higher gradation ofdisplays, there is a problem in that the number of pixels and gradationsis limited because the power consumption and price of the system overallmust be limited.

The voltages used in the circuit blocks on the display device substrate101 are different, and there is therefore a problem in that it isnecessary to jointly use processes that are suited to a plurality ofvoltages, and costs in the manufacturing process increase.

The drive circuit-integrated liquid crystal display devices also have aproblem in that the size of the display device cannot be reduced becausethe controller IC and interface circuit 114 are mounted outside thedisplay device substrate.

In view of the above, the present inventors have filed a patentapplication (Japanese Patent Application (Tokugan) 2004-272638) for aninvention that claims a structure and a drive method for the structurethat advances the integration of circuits on a carrier substrate andintegrates the memory on the carrier substrate. In a circuit in whichMOS (Metal Oxide Semiconductor) transistors with a polysilicon TFT oranother SOI (Silicon on Insulator) structure is integrated, thetechnology allows operation malfunctions due to hysteresis to be limitedand the sensitivity of latch circuits and latch sense amp circuits thathave these MOS transistors as constituent components to be improved.

In this manner, the prior application achieves the initial objects, butin a structure in which memory is integrated on the carrier substrate,it is difficult to reduce the parasitic capacitance of the bit lines.Therefore, there is a limit to reducing the capacitance of the memorycells and it is difficult to reduce the circuit surface area of theframe memory. As a result, it is difficult to reduce the size of displaydevices that use the frame memory.

Also, this technology requires a considerable amount of electric currentfor charging and discharging because the capacitance of the memory cellsis high. Electric potential is reduced due to wiring resistance becausethe circuit surface area is large, and power consumption is high due tocharging and discharging of the parasitic capacitance of the wiring,leading to a limitation to the amount by which power consumption can bereduced.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a display device and anapparatus having the display device, in which size of the display devicecan be reduced because the controller IC and interface circuit areformed on the same substrate as the display device substrate, thecircuit surface area can be made smaller and power consumption can bereduced by decreasing the capacitance of the memory cells, theoperational reliability of the memory unit can be increased, and costscan be made lower, in any cases when the display data of all the pixelsis sent serially to the liquid crystal module with high-speed transferand without high-speed transfer for each frame time interval.

The display device of the present invention is a display device in whicha data-holding circuit having a capacitance, and a display portionhaving a plurality of pixel electrodes are formed on a single firstcarrier substrate, wherein a second carrier substrate disposed oppositethe first carrier substrate is placed above the display portion, but theopposing substrate is not present above the area in which thedata-holding circuit is disposed. In the present invention, theparasitic capacitance of the data-holding circuit is low because thereis no carrier substrate facing the data-holding circuit. As a result,the capacitance of the data-holding circuit can be reduced in the caseof a fixed ratio between the parasitic capacitance and the capacitanceof the memory cell and the like within the data-holding circuit. Sincethe capacitance of the data-holding circuit is small, the area requiredfor the layout of the data-holding circuit is also small.

The display device of another aspect of the present invention is adisplay device in which a data-holding circuit having a capacitance anda display portion having a plurality of pixel electrodes are formed on asingle first carrier substrate, wherein a second carrier substratedisposed opposite the first carrier substrate is provided, and anelectroconductive film is not present in the area, disposed opposite thedata-holding circuit, on the surface of the second carrier substratethat faces the first carrier substrate side. In the present invention,the parasitic capacitance of the data-holding circuit is low becausethere is no electroconductive film on the carrier substrate disposedopposite the data-holding circuit.

In this display device, a light-blocking film formed by using anonconductive body may be disposed in the area, disposed opposite thedata-holding circuit, on the surface of the second carrier substratethat faces the first carrier substrate. Conventionally, when an opticalleakage current is generated, the electric charge stored in thecapacitance of the data-holding circuit is reduced by the leakagecurrent generated in the data-holding circuit, and data erasure or thelike occurs, but errors caused by light entering the data-holdingcircuit can be prevented by a nonconductive light-blocking film. Also,since there is no leakage of electric charge due to optical leakagecurrent, power consumption can be considerably reduced because theapplication of high voltage, short-cycle refreshing, and otheroperations often used as a countermeasure to leakage are not required.Also, the light-blocking film is a nonconductive body, and the parasiticcapacitance of the data-holding circuit is therefore small.

The display device of another aspect of the present invention is adisplay device in which a data-holding circuit having a capacitance, anda display portion having a plurality of pixel electrodes are formed on asingle first carrier substrate, wherein a second carrier substratedisposed opposite the first carrier substrate is provided, and a mediumwhose dielectric constant and dielectric anisotropy vary in accordancewith the frequency is disposed between the second carrier substrate andthe area of the first carrier substrate in which the data-holdingcircuit is disposed.

The medium may be the same material as the display medium of the displayportion.

The medium whose dielectric constant and dielectric anisotropy vary inaccordance with the frequency is preferably a medium in which thedielectric constant is reduced in association with an increase infrequency.

An electroconductive film may be provided to the area, disposed oppositethe data-holding circuit, on the surface of the second carrier substratethat faces the first carrier substrate side.

The data-holding circuit having a capacitance may have a sense amplifiercircuit for amplifying and latching the magnitude of the electricpotential between two nodes.

The sense amplifier circuit preferably has a first and second latchcircuit, and a transmission control portion is provided that allows ordisallows signal transmission between one of the two nodes and at leastone of the latch circuits of the first and second latch circuits.

The output voltage amplitude of the first latch circuit is preferablylower than the output voltage amplitude of the second latch circuit. Ina sense amplifier circuit with such a configuration, the capacitance inthe data-holding circuit can be reduced from ordinary levels because theread signal from the data-holding circuit can be amplified in twostages. In other words, the capacitance in the data-holding circuit canbe greatly reduced by the effect of reducing the capacitance with theaid of a sense amplifier, and the synergistic effect of reducing thecapacitance by reducing the parasitic capacitance. The voltage appliedto the capacitance can also be reduced. As a result, power consumptioncan be considerably cut back. Also, since a sense amplifier thatamplifies in two stages is used, the minimum read voltage width can bereduced and stable operation can be achieved even if the characteristicsfluctuate or otherwise vary.

The dielectric body of the capacitance of the data-holding circuit andthe dielectric body of a storage capacitor provided to the pixel of thedisplay portion are preferably formed with the same film. An increase inthe number of processes can thereby be limited.

A drive circuit for driving the display portion may be disposed on thecarrier substrate on which the data-holding circuit and the displayportion are disposed.

The dielectric body of the capacitance in the data-holding circuit andthe dielectric body of a storage capacitor provided to the pixel arepreferably formed with the same film as at least one of the gate oxidefilms of the transistor having the drive circuit. An increase in thenumber of processes can thereby be considerably limited. A highperformance display device with an internally mounted data-holdingcircuit can thereby be provided at low cost.

The data-holding circuit may be composed of a DRAM in which memory cellscan be refreshed only in a read operation.

The retention time of the memory cells in the DRAM is preferably longerthan the repetition time of the read operation.

The source and drain of at least one transistor in the data-holdingcircuit preferably has an LDD structure.

The apparatus of the present invention has the above-described displaydevice, an electroconductive layer is disposed around the periphery ofthe display device, and the distance between the data-holding circuitand the electroconductive layer around the periphery of the displaydevice is more than 100 times greater than the thickness of thedielectric body in the capacitance of the data-holding circuit. Becauseof this configuration, the parasitic capacitance between theelectroconductive layer and the data-holding circuit is low. Thecapacitance of the data-holding circuit can thereby be reduced.

The distance between the data-holding circuit and the electroconductivelayer around the periphery of the display device is preferably more than1,000 times greater than the thickness of the dielectric body in thecapacitance of the data-holding circuit. Because of this configuration,the parasitic capacitance between the electroconductive layer of theapparatus and the data-holding circuit is low. The capacitance of thedata-holding circuit can thereby be reduced. When the capacitance in thedata-holding circuit is not reduced, the ratio to the parasiticcapacitance is increased, and the effect of the parasitic capacitance isnegligible. Highly stable operation can be achieved as a result.

The apparatus of the present invention has the above-described displaydevice, a dielectric layer is disposed between the display device andthe inner wall of the apparatus, and the dielectric layer is composed ofa low-k material (material with a low dielectric constant). Because ofthis configuration, the parasitic capacitance between theelectroconductive layer of the apparatus and the data-holding circuitcan be reduced, and the capacitance of the data-holding circuit canthereby be reduced.

Another apparatus of the present invention has the above-describeddisplay device, a dielectric layer is disposed between the displaydevice and the inner wall of the apparatus, and the dielectric layer iscomposed of air. Because of this configuration, the parasiticcapacitance between the electroconductive layer of the apparatus and thedata-holding circuit can be reduced, and the capacitance of thedata-holding circuit can thereby be reduced. Since the capacitance canbe reduced, power consumption required for charging and discharging thecapacitance can be considerably reduced as well.

In accordance with the present invention, the capacitance of thedata-holding circuit itself can be reduced by decreasing the parasiticcapacitance of the data-holding circuit, and the area required for thelayout of the circuit can be reduced as a result. The effect of lightincident on the circuit is prevented by providing a nonconductivelight-blocking film, and there is no leakage of electric charge due tooptical leakage current. Therefore, power consumption can beconsiderably reduced because the application of high voltage,short-cycle refreshing, and other operations often used as acountermeasure to leakage are not required. Also, since ratio ofcapacitance in the data-holding circuit and the parasitic capacitancebetween the electroconductive layer of the apparatus and thedata-holding circuit is large, there is little effect from the parasiticcapacitance between the electroconductive layer of the apparatus and thedata-holding circuit, and very stable operation can be achieved.Moreover, an increase in the number of processes can be limited byforming the capacitance of the data-holding circuit with the same filmas the gate oxide film of the transistors of the drive circuit thatdrives the display area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of the display system ofthe common drive circuit-integrated liquid crystal display device of theprior art;

FIG. 2 is a block diagram showing the structure of the display system ofa conventional drive circuit-integrated liquid crystal display devicewith an internally mounted DAC circuit;

FIG. 3A is a perspective view showing the display device of the firstembodiment of the present invention, and FIG. 3B is a cross-sectionaldiagram along line A-A′ of FIG. 3A;

FIG. 4A is a perspective view showing the display device of the secondembodiment of the present invention, and FIG. 4B is a cross-sectionaldiagram along line A-A′ of FIG. 4A;

FIG. 5A is a perspective view showing the display device of the thirdembodiment of the present invention, and FIG. 5B is a cross-sectionaldiagram along line A-A′ of FIG. 5A;

FIG. 6 is a schematic diagram showing the crossover phenomenon ofdielectric anisotropy in a dual-frequency driven liquid crystal;

FIG. 7A is a perspective view showing the display device of the sixthembodiment of the present invention, and FIG. 7B is a cross-sectionaldiagram along line A-A′ of FIG. 7A;

FIG. 8A is a perspective view showing the display device of the eighthembodiment of the present invention, and FIG. 8B is a cross-sectionaldiagram along line A-A′ of FIG. 8A;

FIG. 9 is a circuit diagram of a single bit line of the senseamplifier-containing memory cell array 121;

FIG. 10 is a block diagram showing an example of the frame memorystructure;

FIG. 11 is a circuit diagram showing the upper portion of a bit linecircuit of the display device of the ninth embodiment of the presentinvention;

FIG. 12 is a circuit diagram showing the lower portion of a bit linecircuit of the display device of the ninth embodiment of the presentinvention;

FIG. 13 is a schematic diagram showing the structure of a memory cellwith a single transistor/single capacitance structure;

FIG. 14 is the measurement result of the retention time of thecapacitance of the memory cell shown in FIG. 13;

FIG. 15 is a graph showing the expected relationship between the ratioof d to t and the ratio between the circuit surface area of the DRAMportion (data-holding circuit 3) of the first comparative example andthe circuit surface area of the DRAM portion (data-holding circuit 3) ofthe present invention; and

FIG. 16 is a block diagram showing the operation of a DRAM circuit ofthe display device of the fourth example of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described in detail belowwith reference to the attached diagrams. FIG. 3A is a perspective viewshowing the display device of the first embodiment of the presentinvention, and FIG. 3B is a cross-sectional diagram along line A-A′ ofFIG. 3A. A display area 4 and a data-holding circuit 3 provided with acapacitance (not shown) are aligned in parallel at regular intervals ona first carrier substrate 1. A spacer 5 is formed with greater thicknessthan that of the display area 4 along the side nearest to thedata-holding circuit 3 of the display area 4 and the side facing theretowithout making contact with the data-holding circuit 3 and without beingpresent further outward from the edge portion of the first carriersubstrate 1.

A second carrier substrate 2 facing the first carrier substrate 1 isdisposed above the display area 4, and a fixed interval is provided bythe spacer 5 between the first carrier substrate and the second carriersubstrate. However, the opposing substrate is not present above the areain which the data-holding circuit 3 is disposed.

In a display device configured in this manner, parasitic capacitance isnot present between the capacitance of the data-holding circuit 3 andthe facing carrier substrate because the facing carrier substrate thatis connected in series or in parallel with the capacitance of thedata-holding circuit 3 is not present. It is for this reason that theparasitic capacitance that accompanies the capacitance of thedata-holding circuit 3 is low. As a result, the capacitance of thedata-holding circuit 3 can be reduced in the case of a low ratio betweenthe capacitance of the data-holding circuit 3 and the parasiticcapacitance that accompanies the data-holding circuit 3 is fixed. Sincethe capacitance of the data-holding circuit 3 can be reduced, the arearequired for the layout of the data-holding circuit 3 is small.

The second embodiment of the present invention is described next. FIG.4A is a perspective view showing the display device of the secondembodiment of the present invention, and FIG. 4B is a cross-sectionaldiagram along line A-A′ of FIG. 4A. In FIG. 4, the same referencenumerals are assigned to the same constituent elements as FIG. 3, and adetailed description thereof is omitted. In the structure of the firstembodiment described above, a second carrier substrate 2 disposedopposite the first carrier substrate 1 is placed above the display area4, and the opposing substrate is not present above the area in which thedata-holding circuit 3 is present. In the present invention, however,the second carrier substrate 2 facing the first carrier substrate 1above the display area 4 is disposed so as to also be present above thedata-holding circuit 3, but is otherwise the same structure as theabove. Of the portions that intersect with the data-holding circuit 3 ofthe second carrier substrate 2 in the plan view, an electroconductivefilm (not shown) may be present on the surface that is on the oppositeside from the surface facing the data-holding circuit 3.

In a display device configured in such a manner, parasitic capacitancebetween the capacitance of the data-holding circuit 3 and the seriallyor parallelly connected opposite carrier substrate 2 is lower than whenan electroconductive film is present in the area, disposed opposite thedata-holding circuit 3, on the surface of the second carrier substrate 2disposed opposite the data-holding circuit 3. It is for this reason thatthe parasitic capacitance that accompanies the capacitance of thedata-holding circuit 3 is low. As a result, the capacitance of thedata-holding circuit 3 can be reduced in the case of a fixed ratiobetween the capacitance of the data-holding circuit 3 and the parasiticcapacitance that accompanies the capacitance of the data-holding circuit3. Since the capacitance of the data-holding circuit 3 can be reduced,the area required for the layout of the data-holding circuit 3 is small.

The third embodiment of the present invention is described next. FIG. 5Ais a perspective view showing the display device of the third embodimentof the present invention, and FIG. 5B is a cross-sectional diagram alongline A-A′ of FIG. 5A. In FIG. 5, the same reference numerals areassigned to the same constituent elements as FIGS. 3 and 4, and adetailed description thereof is omitted. In the second embodimentdescribed above, a second carrier substrate 2 disposed opposite thefirst carrier substrate 1 is provided so as to be present above thedata-holding circuit 3 as well, but in the present embodiment, thestructure is different in that a nonconductive light-blocking film 6 isprovided to the area, disposed opposite the data-holding circuit 3, onthe surface of the second carrier substrate 2 that faces thedata-holding circuit 3. Other aspects of the structure are the same. Thelight-blocking film 6 can have a function for lessening the intensity ofthe transmitted light, and the material, film thickness, and otheraspects of the film structure are not particularly limited as long asthe film is a nonconductive body. The film may perform thelight-blocking function by absorbing light that is about to pass throughor by reflecting the light.

In a display device configured in such a manner, optical leakage currentand other adverse effects are unlikely to occur even if an area of thedata-holding circuit 3 is irradiated with light from the side facing thesecond carrier substrate 2. Also, if for any reason optical leakagecurrent occurs, the magnitude of the leakage current is far less thanwhen a light-blocking film 6 is not present. Since the light-blockingfilm 6 is formed from a nonconductive body, the parasitic capacitancethat accompanies the capacitance of the data-holding circuit 3 is alsolow. As a result, the capacitance of the data-holding circuit 3 can bereduced in the case of a fixed ratio between the capacitance of thedata-holding circuit 3 and the parasitic capacitance that accompaniesthe capacitance of the data-holding circuit 3. Since the capacitance ofthe data-holding circuit 3 can be reduced, the area required for thelayout of the data-holding circuit 3 is small.

The fourth embodiment of the present invention is described next. In thesecond embodiment described above, only a data-holding circuit 3, adisplay area 4, and a spacer 5 are present between the first carriersubstrate 1 and second carrier substrate 2, but in the presentembodiment, the structure is different in that a medium whose dielectricconstant and dielectric anisotropy vary in accordance with the frequencyis present between the first carrier substrate 1 and the second carriersubstrate 2 in the vicinity of the data holding circuit 3. Other aspectsof the structure are the same.

It is particularly preferred that the medium that is present between thefirst carrier substrate 1 and second carrier substrate 2 in the vicinityof the data-holding circuit 3 have a feature in which the dielectricconstant decreases in accordance with the frequency. When the medium hasdielectric anisotropy, the medium preferably exhibits considerablevariation as the sign of the dielectric anisotropy changes due to thefrequency. An example of such a material is a liquid crystal substancethat is referred to as a dual-frequency driven liquid crystal or thelike.

This material has positive dielectric anisotropy under a low frequency,and has negative dielectric anisotropy under a high frequency. In otherwords, a phenomenon referred to as the “crossover phenomenon” occurswhereby the sign of the dielectric constant is reversed at a certainfrequency (referred to as the “crossover frequency”). As a result, thevalue of the dielectric constant is different under low and highfrequencies, and the dielectric constant is lower under high frequency.FIG. 6 shows the relationship between the frequency and the dielectricanisotropy of such a material. The sign of the dielectric anisotropy ofsuch a material varies under high frequency, and the absolute valuethereof is ordinarily low in comparison with the value obtained underlow frequency.

The crossover frequency is different depending on the material but isgenerally several MHz (megahertz). For example, when the access signalto the data-holding circuit 3 is 1 MHz or greater, it is in a domain inwhich the dielectric constant of the medium begins to decrease, anelectric field between other wiring on the same substrate is generatedby the access signal, and the dielectric constant in the direction ofthe electric field decreases. It is for this reason that the dielectricconstant of the medium on the data-holding circuit 3 is reduced when thedata-holding circuit 3 is operating. As a result, the parasiticcapacitance that accompanies the capacitance of the data-holding circuit3 is reduced.

The fifth embodiment of the present invention is described next. In thethird embodiment described above, a nonconductive light-blocking film 6is disposed in the area facing the data-holding circuit 3 on the surfaceof the second carrier substrate 2 that faces the data-holding circuit 3,and only the data-holding circuit 3, display area 4, spacer 5, andnonconductive light-blocking film 6 are present between the firstcarrier substrate 1 and second carrier substrate 2. In the presentembodiment, however, the structure is different in that, in addition tothe above components, a medium in which the dielectric anisotropy anddielectric constant vary in accordance with the frequency is presentbetween the first carrier substrate 1 and second carrier substrate 2 inthe vicinity of the data-holding circuit 3, in the same manner as in thefourth embodiment. Other aspects of the structure are the same.

Based on this structure, optical leakage current and other adverseeffects are unlikely to occur even if an area of the data-holdingcircuit 3 is irradiated with light from the side facing the secondcarrier substrate 2, in the same manner as in the third embodimentdescribed above. Also, if for any reason optical leakage current occurs,the magnitude of the leakage current is far less than when alight-blocking film 6 is not present. Since the light-blocking film 6 isformed from a nonconductive body, the parasitic capacitance thataccompanies the capacitance of the data-holding circuit 3 is also low.As a result, the capacitance of the data-holding circuit 3 can bereduced in the case of a fixed ratio between the capacitance of thedata-holding circuit 3 and the parasitic capacitance that accompaniesthe data-holding circuit 3. Since the capacitance of the data-holdingcircuit 3 can be reduced, the area required for the layout of thedata-holding circuit 3 is small.

In the same manner as the fourth embodiment described above, theparasitic capacitance that accompanies the capacitance of thedata-holding circuit 3 can be reduced by selecting a dual-frequencydriven liquid crystal having a crossover frequency of several megahertzas the medium that is disposed between the first carrier substrate 1 andsecond carrier substrate 2 in the vicinity of the data-holding circuit3.

The sixth embodiment of the present invention is described next. FIG. 7Ais a perspective view showing the display device of the sixth embodimentof the present invention, and FIG. 7B is a cross-sectional diagram alongline A-A′ of FIG. 7A. In FIG. 7, the same reference numerals areassigned to the same constituent elements as FIGS. 3 to 5, and adetailed description thereof is omitted. In the fifth embodimentdescribed above, a nonconductive light-blocking film 6 is disposed inthe area facing the data-holding circuit 3 on the surface of the secondcarrier substrate 2 that faces the data-holding circuit 3. In thepresent embodiment, however, the structure is different in that anelectroconductive film 7 is provided rather than a nonconductivelight-blocking film 6. Other aspects of the structure are the same.

In a display device configured in such a manner, when an electric fieldoccurs between the data-holding circuit 3 disposed on the first carriersubstrate 1 and the electroconductive film 7 disposed on the secondcarrier substrate 2, the dielectric constant in the direction of theelectric field is reduced by providing a dual-frequency driven liquidcrystal having a crossover frequency of several megahertz in cases inwhich the access signal to the data-holding circuit 3 is 1 MHz orhigher, for example. As a result, the parasitic capacitance thataccompanies the capacitance of the data-holding circuit 3 can bereduced.

The seventh embodiment of the present invention is described next. Afeature of the present embodiment is that the medium used in the fourthto sixth embodiments described above, in which the dielectric constantand dielectric anisotropy vary in accordance with the frequency, isabsent between the first carrier substrate 1 and second carriersubstrate 2 in the vicinity of the data-holding circuit 3, but isdisposed instead between the first carrier substrate 1 and secondcarrier substrate 2 so as to cover the display area 4, and the mediumdoubles as the display medium of the display area 4. The dual-frequencydriven liquid crystal described above is preferably used as the medium.

The drive frequency of the display area 4 is generally several tens toseveral hundred hertz, and is within a range in which the dual-frequencydriven liquid crystal has a high dielectric constant, the dielectricanisotropy is, for example, positive, and the value of the dielectricanisotropy is high. The access signal of the data-holding circuit 3 is,for example, 1 MHz or higher, as described above, and is in a range inwhich the dielectric constant of the dual-frequency driven liquidcrystal is low. For this reason, the average dielectric constant is kepthigh by the high dielectric anisotropy in the display area 4 when thedisplay device of the present invention is operating, but the dielectricconstant is lower in the vicinity of the data-holding circuit 3. Asufficient response can thereby be made to the display signal because ofthe high dielectric anisotropy, and the parasitic capacitance thataccompanies the capacitance of the data-holding circuit 3 is reducedbecause of the low dielectric constant in the data-holding circuit 3. Asa result, excellent display can be stably obtained by the display deviceof the present embodiment.

The eighth embodiment of the present invention is described next. FIG.8A is a perspective view showing the display device of the eighthembodiment of the present invention, and FIG. 8B is a cross-sectionaldiagram along line A-A′ of FIG. 8A. In FIG. 8, the same referencenumerals are assigned to the same constituent elements as FIGS. 3 to 7,and a detailed description thereof is omitted. In the sixth embodimentdescribed above, an electroconductive film 7 is disposed in the areafacing the data-holding circuit 3 on the surface of the second carriersubstrate 2 that faces the data-holding circuit 3. In the presentembodiment, however, the structure is different in that anelectroconductive film 8 is not disposed in the area facing thedata-holding circuit 3 on the surface of the second carrier substrate 2that faces the data-holding circuit 3, but is disposed across the entiresurface on the side opposite from the surface of the second carriersubstrate 2 that faces the first carrier substrate. Other aspects of thestructure are the same. Preferred examples of the electroconductive film8 disposed in this manner include an electroconductive film that is usedin sensor applications for touch panels, and an electroconductive filmthat is used to prevent the external electric field from affecting thedisplay area in the display mode or the like of the IPS(In-Place-Switching) method.

In the present embodiment, it is possible to keep the parasiticcapacitance that accompanies the capacitance of the data-holding circuit3 low because of the thickness of the second carrier substrate 2, evenif an electroconductive film 8 as described above is provided across theentire surface on the side opposite from the surface of the secondcarrier substrate 2 that faces the first carrier substrate.

Described next is the ninth embodiment of the present invention. In theninth embodiment of the present invention, the data-holding circuit 3having the capacitance (not shown) of the display device of the first toeighth embodiments is provided with a sense amplifier circuit thatamplifies and latches the magnitude of the electric potential betweentwo nodes. Data stored in the capacitance of the data-holding circuit 3can easily be read by using the sense amplifier circuit.

FIG. 9 is an example of a circuit diagram of a single bit line of thesense amplifier-containing memory cell array 121. This circuit iscomposed of a pair of bit lines (XB, B), 240 memory cells 161 connectedto two bit lines in an alternating manner, a precharge circuit 162, asense amplifier circuit 160, and other components. The data of the dataline 163 is written in the pair of bit lines (XB, B) that is selected bythe signal from a column decoder 122 at the time of data writing. Thedata written in the pair of bit lines (XB, B) is written in the memorycell 161 of the selected word line ([W 239], W [118], W [1], and W [0]in the illustrated example). Conversely, at the time of data read-out,the data of the selected word line is read to the pair of bit lines (XB,B), is amplified in the sense amplifier circuit 160, and is outputted tothe output register side.

When the data-holding circuit (memory cell 161) based on the capacitoris disposed between a pair of bit lines (XB, B) such at that shown inFIG. 9, the following relations can be derived about the parasiticcapacitance of the pair of bit lines and the operation of the senseamplifier circuit 160. First, the data held in the capacitance of thedata-holding circuit (memory cell 161) during the read operation is readto the pair of bit lines, and the voltage ΔV thus read at this time isgiven in the following EQ. 1, where V_(DD) is the voltage that isexpected to be able amplify the signal, C_(S) is the capacitance of thedata-holding circuit, and C_(b) is the parasitic capacitance of the pairof bit lines.

$\begin{matrix}{{{\Delta \; V}} = {\frac{C_{s}}{2 \cdot \left( {C_{s} + C_{b}} \right)} \cdot V_{DD}}} & \left\lbrack {{EQ}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

When the voltage ΔV read from the pair of bit lines is larger than thesensitivity S_(A) of the sense amplifier circuit 160, a bit line circuitsuch as that shown in FIG. 9 operates normally. In this case, thesensitivity S_(A) of the sense amplifier 160 is the boundary thatdetermines whether the sense amplifier circuit 160 malfunctions or not,and when expressed as a voltage value, the sensitivity is improved asthe value becomes smaller. From this, the relation shown in thefollowing EQ. 2 can be obtained between the capacitance C_(S) of thedata-holding circuit and the parasitic capacitance C_(b) of the pair ofbit lines.

$\begin{matrix}{C_{s} > \frac{2 \cdot S_{A}}{V_{DD} - {2 \cdot S_{A}}}} & \left\lbrack {{EQ}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

That is, if the parasitic capacitance C_(b) of the pair of bit linesbecomes large, the capacitance C_(S) of the data-holding circuit must beincreased, and if the parasitic capacitance C_(b) of the pair of bitlines is reduced, the capacitance C_(S) of a data-holding circuit can bereduced.

FIG. 10 is a diagram showing the memory configuration. The core portionof a frame memory consists of a memory cell array 121 with a senseamplifier, a column decoder 122, and a row decoder 123. The memory cellarray 121 with a sense amplifier can access a specific memory cell byspecifying a row address by using the row decoder 123 and a columnaddress by using the column decoder 122. The data signal thus read fromthe memory cell is output via the sense amplifier. The frame memorycircuit described above is formed on a glass substrate 120.

Described next is the tenth embodiment of the present invention. FIGS.11 and 12 are block diagrams showing the structure of the bit linecircuit of the present embodiment. For convenience of illustration, thestructure is divided into two diagrams, but a single bit line circuit isformed by connecting the points J with each other and the points K witheach other, as shown in FIGS. 11 (upper portion of the DRAM circuitdiagram) and 12 (lower portion of the DRAM circuit diagram).

In the ninth embodiment described above, the data-holding circuit 3having a capacitance (not shown) is provided with a single senseamplifier circuit that amplifies and latches the magnitude of theelectric potential between two nodes. In the present embodiment,however, the bit line circuit is composed of a sense amplifier circuithaving a two stage configuration. More specifically, the sense amplifiercircuit is composed of a low-amplitude preamp circuit 4902 that is afirst circuit for amplifying the electric potential difference betweenthe nodes to a relatively small amplitude value, and a full-swingamplifier circuit 4903 that is a second circuit for amplifying theelectric potential difference obtained by the low-amplitude preampcircuit to the originally required amplitude value.

Switches M03 and M04 are turned off and the low-amplitude preamp circuit4902 is separated from the bit line before the full-swing amplifiercircuit 4903 is operated so that voltage is not applied during fullswing to the elements (4901 a and 4901 b) that form the low-amplitudepreamp circuit 4902. Thus, the voltage applied to the element thatconstitutes the low-amplitude preamp circuit 4902 is kept low becausethe structure is composed of a low-amplitude preamp circuit 4902 andfull-swing amplifier circuit 4903, and a high voltage amplified by thefull-swing amplifier circuit 4903, that is to say, the ultimatelyrequired output voltage, is driven so as not to be not applied to thelow-amplitude preamp circuit 4902.

Furthermore, the sensitivity of the sense amplifier can be improved inthe present embodiment because the configuration has a two-stageconfiguration composed of the low-amplitude preamp circuit 4902 andfull-swing amplifier circuit 4903. S_(A) is ordinarily determined by thesensitivity of the full-swing amplifier circuit 4903. However, thepresence of the low-amplitude preamp circuit 4902 in the presentembodiment allows sensitivity having a lower value than the sensitivityof the full-swing amplifier circuit 4903 to serve as S_(A). In otherwords, it is possible for the value of S_(A) to become small in EQ. 2 inthe present embodiment, and to make the capacitance C_(S) of adata-holding circuit to be less than that of the ninth embodiment. Thus,the present embodiment is configured so that the parasitic capacitanceC_(b) of the pair of bit lines can be reduced by the effect of thetwo-stage sense amplifier circuit, and since the capacitance with thesecond carrier substrate 2 is kept low, the parasitic capacitance thataccompanies the capacitance of the data-holding circuit 3 can bereduced.

The eleventh embodiment of the present invention is described next. Inthe present embodiment, a drive circuit is disposed on the first carriersubstrate 1 having a data-holding circuit 3 and display area 4. Also,the dielectric body constituting the capacitance of the data-holdingcircuit 3 and the dielectric body that forms the storage capacitor ofthe pixels of the display area 4 are formed with the same film as thegate insulation film of the transistor that forms the drive circuit.There is therefore no need to adopt a special process in order to formthe capacitance of the data-holding circuit 3. It is for this reasonthat the display device of the present invention can be formed with lowcost.

The twelfth embodiment of the present invention is described next. Thepresent embodiment uses a DRAM (Dynamic Random Access Memory) in whichthe memory cells used in the first to eleventh embodiments are used,wherein the cells can be refreshed solely by the operation of readingfrom the data-holding circuit 3. In order to allow the memory cell torefresh solely by the read operation, a memory cell is used that has aretention time (hold interval, storage interval) that is longer than therepeat interval of the read operation. A double-sided LDD (Lightly DopedDrain) structure is preferably used in a portion of the transistor ofthe memory cell. Particularly preferred is the use of the double-sidedLDD structure in the transistor that is connected to the memorycapacitance. The retention time of the memory cell capacitance canthereby be extended.

In accordance with the twelfth embodiment of the present invention, thenumber of refresh operations can be considerably reduced in contrast toa conventional DRAM. When a single read operation requires 16.7milliseconds, for example, only one refresh operation can be performedin the 16.7 milliseconds. The power consumption associated with therefresh operation can be considerably reduced and a low powerconsumption display device can thereby be obtained.

Also, leak current can be considerably limited by using the double-sidedLDD structure in which both the source side and drain side of thetransistor have LDD structures. The retention time is extended as aresult. The structure of the LDD in the double-sided LDD structure maybe a simple LDD structure or a GOLD (Gate Overlapped LDD) structure inwhich the gate electrode overlaps the LDD area.

FIG. 14 shows an example of the results of measuring the retention timeof the capacitance in the memory cell of the present invention, and moreparticularly a memory cell with a single transistor/single capacitancestructure. FIG. 13 is a schematic diagram showing the structure of thememory cell which was used for the measurement. This memory cell isconfigured with a single transistor and a single capacitance, and a wordline is connected to the transistor gate (this node is notated asN_(W)). A bit line is connected to the side that is not connected to thecapacitance of the source or drain of the transistor (this node isnotated as N_(B)). The symbol N_(S) is the node between the transistorand the capacitance. Shown in the measurement in FIG. 15 is thevariation over time of the voltage of N_(S) when N_(S)=5V and N_(B)=0V,and the voltage of N_(S) when N_(S)=0V and N_(B)=5V, when N_(W)=0V. Itis apparent from FIG. 10 that the retention time in this structure issufficiently longer than the read operation time of 16.7 milliseconds ofan ordinary display. As a result, data can be saved even if a functionis provided for refreshing the memory cell solely during the readoperation.

The thirteenth embodiment of the present invention is described next.The present embodiment relates to an apparatus in which the displaydevice of the first to twelfth embodiments described above is used. Whenthe display device is used in such an apparatus, an electroconductivelayer may be disposed in the apparatus around the periphery of thedevice. There are also situations in which all or a part of the insideof the apparatus case is metal plated, or a thin copper foil is used asan electromagnetic shield in some areas. In such a situation, thedistance between the capacitance of the data-holding circuit 3 and theelectroconductive layer on the periphery of the display device of theapparatus is preferably more than 100 times greater than the thicknessof the dielectric body of the capacitance of the data-holding circuit 3.The distance is more preferably 1,000 times greater. The parasiticcapacitance between the apparatus and the data-holding circuit 3 canthereby be reduced and the circuit surface area can be made smaller.

The fourteenth embodiment of the present invention is described next.The present embodiment relates to an apparatus in which the displaydevice of the first to twelfth embodiments described above is used. Inorder to fix in place the inner wall and display device of the apparatusor to give buffer strength against shock, a dielectric layer may beprovided between the display device and apparatus. In other words, thedielectric layer is an adhesive that fixes the display device andapparatus in place, or is a cushioning material between the displaydevice and the inner wall of the apparatus. In this case, a low-kmaterial is used in the above-described dielectric layer. A low-kmaterial is a material having a low dielectric constant, and the pointof reference for a low dielectric constant is ordinarily one that isless than the dielectric constant of a silicon oxide film (SiO₂). Thedielectric constant of a silicon oxide film is 4.2 in a quartz crystal,and is ordinarily about 3.8 in film formation by plasma CVD (ChemicalVapor Deposition). For this reason, a material whose dielectric constantis less than 4 is generally referred to as a low-k material.

In a display device configured in this manner, the parasitic capacitancebetween the apparatus and the data-holding circuit 3 can be reduced andthe circuit surface area can be made smaller.

The fifteenth embodiment of the present invention is described next. Thepresent embodiment relates to an apparatus in which the display deviceof the first to twelfth embodiments described above is used. In thepresent embodiment, the dielectric layer described in the fourteenthembodiment described above is not a low-k material, but is composed ofair. The dielectric constant of an air layer is about 1 and is very low.

In a display device configured in this manner, the parasitic capacitancebetween the apparatus and the data-holding circuit 3 can be reduced andthe circuit surface area can be made smaller.

Described next is the first example that shows the effects of thepresent invention. In the present example, a glass substrate was used asthe first carrier substrate 1 of the display device of the thirdembodiment of the present invention, and the display area 4 anddata-holding circuit 3 were fabricated using a TFT array composed ofpolysilicon (polycrystalline silicon, poly-Si).

More specifically, a silicon oxide film was formed on a glass substrateas the first carrier substrate 1, after which amorphous silicon wasgrown. Next, an excimer laser was used to anneal the amorphous siliconand form polysilicon, and a 100-Å (10 nm) silicon oxide film was grown.

A silicon film formed according to the above-mentioned process on theglass substrate was patterned with a desired shape, after which aphotoresist was patterned, and the source and drain areas were formed bydoping with phosphorus ions. A silicon oxide film having a thickness of900 Å (90 nm) was grown, and microcrystal silicon (μ-c-Si) and tungstensilicide (WSi) were thereafter grown and patterned in the form of agate. A silicon oxide film and silicon nitride film were consecutivelygrown, connector holes were thereafter formed, an aluminum film andtitanium film were formed by sputtering and the product was patterned. Asilicon nitride film was formed, connector holes were thereafter formed,and ITO (Indium tin Oxide), which is a transparent electrode, was formedas a pixel electrode and patterned. A planar TFT pixel switch was formedin this manner and a TFT array was formed.

The peripheral circuit portion was provided with an n-channel TFT in thesame manner as in the pixel switch formation method described above, anda p-channel TFT in which the p-channel was formed by ion doping bysubstantially the same method as the n-channel TFT.

The data-holding circuit 3 was a DRAM formed with a TFT, and a singlememory cell was formed with a single transistor and a singlecapacitance. The memory cell was connected to a bit line and a wordline. A memory cell array composed of a memory cell and a pair of bitlines was formed by alternately arranging such memory cells between twobit lines.

A 4-μm patterned column was formed along the side nearest to thedata-holding circuit 3 of the display area 4 on the TFT substrate formedin the above-described step and along the opposing side thereof, and thecolumn was designed to have shock resistance while simultaneously actingas a spacer 5 for maintaining a cell gap.

In the second carrier substrate 2, an ITO surface was patterned in thearea disposed opposite a display area (pixel area) 4 when set to facethe TFT substrate, a light-blocking film 6 was provided in the form of anonconductive light-absorbing resin in the area disposed opposite theDRAM portion (data-holding circuit 3), and a UV-curing sealing materialwas applied in the other areas on the same surface.

Liquid crystal was fed dropwise through a dispenser in the display area4 of the TFT substrate, the TFT substrate and second carrier substrate 2were brought together, and the sealing portion was irradiated with UVrays to perform bonding. A nematic liquid crystal was used as the liquidcrystal material, and a TN-type configuration was formed by adding achiral material and matching the rubbing direction.

In the present example, an excimer laser was used to form a polysiliconfilm, but it is also possible to use, for example, a continuouslyoscillating CW (Continuous Wave Oscillation) laser or another laser.

Described next is a second example that shows the effects of the presentinvention. In the present example, a glass substrate was used as thefirst carrier substrate 1 of the display device of the first embodimentof the present invention in the same manner as in the first example, anda display area 4 and data-holding circuit 3 were fabricated with a TFTarray composed of polysilicon (polycrystalline, poly-Si).

The present embodiment involves a structure in which a second carriersubstrate 2 is not present above the DRAM portion (data-holding circuit3). When the second carrier substrate 2 was set so as to face the TFTsubstrate, an ITO surface was patterned in the area facing the displayare 4, and a 4-μm resin spacer was formed in the area disposed oppositethe peripheral portion of the display area 4 of this surface.

A thermosetting sealing material was applied to the external peripheryof the display area 4 of the TFT substrate formed by the same step as inexample 1.

The TFT substrate and second carrier substrate 2 were bonded by applyingheat, after which liquid crystal was injected into the gap provided bythe resin spacer 5. A nematic liquid crystal was used as the liquidcrystal, and a TN-type configuration was formed by adding a chiralmaterial and matching the rubbing direction.

To draw a comparison with example 2 described above, a display devicewas fabricated with a second carrier substrate having ITO in the areadisposed opposite the DRAM portion (data-holding circuit 3) of the TFTsubstrate. The resulting structure is comparative example 1. In example2 of the present invention and comparative example 1, the capacitance ofthe data-holding circuit 3 is optimized for each case.

Comparing example 2 of the present invention and comparative example 1,the parasitic capacitance per 1 micron of a bit line is reduced by 0.25fF, that is, 16% in example 2, in contrast to 0.30 fF in comparativeexample 1. Because of this effect, the layout length W of the memorycell array portion drops to 2.3 mm in example 2, in contrast to 4.4 mmin comparative example 1. The reduction ratio exceeds 47%.

The reason that the layout length of the memory cell array differs moregreatly than the difference in parasitic capacitance is that therequired memory cell capacitance is considerable when the parasiticcapacitance is large, the length of the bit lines is extended, theoverall parasitic capacitance of the bit lines increases when the lengthof the bit lines is extended, and the required memory cell capacitanceincreases. Therefore, the layout length of the memory cell array portionis determined by making the memory cell capacitance, the bit linelength, and the parasitic capacitance per 1 micron of bit line intoparameters in the design and optimizing the parameters. As describedabove, a considerable difference in the layout length of the memory cellarray portion is created between example 2 and comparative example 1 asa result of such optimization.

Described next is the third example of the present invention. In example1 described above, a film was not provided to the surface on the sideopposite from the surface of the second carrier substrate 2 that facesthe TFT substrate. In the present example, however, the structure isdifferent in that an electroconductive film 8 is provided to the surfaceon the side opposite from the surface of the second carrier substrate 2that faces the TFT substrate. Other aspects of the structure are thesame. Even with such a configuration, a circuit surface area that isless than in the comparative example 1 described above was obtained inthe display device of the present example.

FIG. 15 shows the expected relationship between the ratio of d to t andthe ratio between the circuit surface area of the DRAM portion(data-holding circuit 3) of the comparative example 1 and the circuitsurface area of the DRAM portion (data-holding circuit 3) of the presentinvention, wherein d is the distance between the DRAM portion(data-holding circuit 3) and the electroconductive film 7 or 8(regardless of whether it is disposed on the surface facing thedata-holding circuit 3 or on the surface opposite from the side facingthe data-holding circuit 3) disposed on the second carrier substrate 2,and t is the thickness of the dielectric layer of the capacitance of theDRAM portion (data-holding circuit 3).

It is apparent from FIG. 15 that the circuit surface area of the DRAMportion (data-holding circuit 3) can be reduced as the distance dincreases between the DRAM portion (data-holding circuit 3) and theelectroconductive film 7 or 8 disposed on the second carrier substrate2, in comparison with the thickness t of the dielectric film of thecapacitance of the DRAM portion (data-holding circuit 3). In particular,when the ratio t:d exceeds 1:100, the circuit surface area of the DRAMportion (data-holding circuit 3) can be reduced to ¾ or less of that ofcomparative example 1, and when the ratio t:d exceeds 1:1000, thecircuit surface area reduction essentially reaches a limit, and aconfiguration with the smallest circuit surface area is made possible.

More specifically, when the thickness of the dielectric film of thecapacitance of the DRAM portion (data-holding circuit 3) is 200 nm, forexample, the smallest circuit surface area can be achieved by settingthe distance from the electroconductive film 7 or 8 of the secondcarrier substrate 2 to be 200 μm or greater. Also, when the thickness ofthe dielectric film of the capacitance of the DRAM portion (data-holdingcircuit 3) is 50 nm, the smallest circuit surface area can be achievedby setting the distance from the electroconductive film 7 or 8 of thesecond carrier substrate 2 to be 50 μm or greater.

In the case that a thin substrate is used as the second carriersubstrate 2, and the substrate thickness is, for example, less than 50μm, the thickness of the dielectric film of the capacitance of the DRAMportion (data-holding circuit 3) can be made less than 50 nm when thegoal is to obtain the minimal circuit surface area. Such a thinsubstrate can be obtained by abrasion or other treatment that involvespolishing, treatment with hydrofluoric acid or another chemical liquid,or laser abrasion of the glass substrate. Also, a plastic substrate maybe used.

In the converse case that the thickness of the dielectric film of thecapacitance of the DRAM portion (data-holding circuit 3) is 200 nm, forexample, a space that is 200 μm or greater is provided between the DRAMportion (data-holding circuit 3) and the physical structure of theapparatus when the display device of the present invention is disposedwithin the apparatus, and the smallest circuit surface area can beachieved by disposing no electroconductive body in this space.

Described next is the fourth example of the present invention. In thepresent example, the ninth embodiment is implemented in which a senseamplifier with a two-stage configuration is used on the basis of thefirst embodiment of the present invention. The method of forming a TFTsubstrate is the same as the method described above in example 1. Inparticular, the circuit configuration and operation of the DRAM portion(data-holding circuit 3) are described below.

The configuration of the bit line circuit of the present example isdescribed with reference to FIGS. 11 and 12. For convenience ofillustration, the structure is divided into two diagrams, but a singlebit line circuit is formed by connecting the points J with each otherand the points K with each other, as shown in FIGS. 11 (upper portion ofthe DRAM circuit diagram) and 12 (lower portion of the DRAM circuitdiagram).

The first circuit; that is, the low-amplitude preamp circuit 4902, andthe second circuit; that is, the full-swing amplifier circuit 4903, areconnected to the pair of bit lines 5301 a, 5301 b. The selected memorycell is connected to the bit line ODD (5301 a) when the word address isan odd number. Shown in the diagram as an example is the case in which amemory cell 5303 composed of an N-channel MOS (Metal OxideSemiconductor) transistor M12 and a capacitance C2 is selected by theword line WL_ODD. Connected in a similar fashion is a memory cellselected by the bit line EVN (5301 b) when the word address is an evennumber. Shown in the diagram as an example is a case in which a memorycell 5304 composed of an N-channel MOS transistor M13 and a capacitanceC1 is selected by the word line WL_EVN. The plurality of other memorycells are omitted.

A precharge circuit 5302 composed of N-channel MOS transistors M14 toM16 is connected to the pair of bit lines, and the MOS transistors areswitched on or off with a signal applied to the PC node. When V_(DD1)/2is applied to the PCS and the control line PC is set to a high level,the pair of bit lines is set to V_(DD1)/2.

A transfer gate consisting of MTG3A and MXTG3A for reading data isconnected to the bit line EVN, and the transfer gate is switched on oroff by the control lines TG3A and XTG3A (a signal with a complimentaryrelationship with TG3A is applied). Also, the transfer gate consistingof MTG3B and MXTG3B is connected to the bit line ODD, and the transfergate is turned on or off by control lines TG3B and XTG3B. The twotransfer gates are activated when data is read to an OUT node. One ofthe transfer gates is switched on in accordance with whether the wordaddress of the memory cell to be read is an odd number or an evennumber.

An analog switch MTG1A is connected to the bit line EVN for datawriting, and this analog switch is switched on or off by the controlline TG1A. An analog switch MTG1B is connected to the bit line ODD, andthis analog switch is switched on or off by the control line TG1B. Thesetwo analog switches are activated when data is to be written. Only oneof the analog switches is switched on in accordance with whether theword address of the memory cell to be written is an even or odd number.

The transfer gate consisting of MDRGT and MXDRGT is switched on or offby a column decoder (not shown). DRGT is switched on during a writeoperation and in a case in which the column address corresponds to thebit line circuit of this address. A data bus signal is transmitted toswitches MTG1A and MTG1B and is written to the bit line by way of one ofthe switches.

The power supply voltage in the present example is V_(DD1). Also, theSAN nodes of the low-amplitude preamp circuit 4902 and the full-swingamplifier circuit 4903 are connected to V_(SS) (=0 V). The SAP node isconnected to V_(DD1). The terminal V_(plate) on the side not connectedto the MOS transistor of the capacitance in the memory cells 5303 and5304 is connected to the V_(DD1)/2, and the voltage stress between thecapacitance terminals is minimized. In FIG. 14, C_(d) indicates theparasitic capacitance of each bit line.

The operation of the DRAM circuit of the present example is describednext with reference to FIG. 16. Described first is the operation in thecase in which data is read to the OUT node from the memory cells 5303and 5304.

The pair of bit lines (ODD, EVN) are pre-charged to V_(DD1)/2 by theprecharge circuit 5302 when the PC is started up at time A. A high levelis applied to PAS at time B, at which the pair of bit lines has beenpre-charged, and the switches M03 and M04 are switched on. The nodes Aand B are thereafter pre-charged to V_(DD1)/2.

A high voltage is then applied to a single word line at time C. In thisexample, the high voltage is applied to WL_EVN. The ΔV voltage isthereby read to the bit line EVN by using the voltage held by C1 of thememory cell 5304. When the voltage held by C1 is V_(DD), the voltagethat appears at the bit line EVN is V_(DD1)/2+|ΔV|, and when the voltageheld by C1 is 0, the voltage that appears is V_(DD1)/2−|ΔV|. The valueof |ΔV| is expressed by EQ. 1 described above.

Described below is the case in which the voltage held by C1 of thememory cell 5304 is V_(DD1), and a voltage of V_(DD1)/2+|ΔV| appears atthe bit line EVN when high voltage is applied to WL_EVN.

At time D, the low-amplitude preamp circuit 4902 starts theamplification and latching operation by applying a high level to SE3.Since the voltage of EVN is V_(DD1)/2+|ΔV| and the voltage of ODD isV_(DD1)/2, the voltage of ODD is reduced to V_(SS) (=0 V) by the sensingoperation of the low-amplitude preamp circuit 4902. On the other hand,the voltage of EVN is reduced only slightly and is about (V_(DD1)/2)−β,for example. In this expression, β is the difference between V_(DD1)/2and the voltage at which the higher-voltage node stabilizes.

When the electric potential difference ΔV of EVN and ODD are amplifiedto the desired electric potential difference by the low-amplitude preampcircuit 4902, and writing to the pair of bit lines (ODD, EVN) iscompleted, PAS is set to a low level as indicated by E, the switches M03and M04 are switched off, and the low-amplitude preamp circuit 4902 isseparated from the pair of bit lines.

A body potential reset pulse for resetting the body potential of M01 andM02 is then applied to the low-amplitude preamp circuit 4902.

On the other hand, the voltage (0, (V_(DD1)/2) −β) amplified by thelow-amplitude preamp circuit 4902 and held in the pair of bit lines isamplified to (0, V_(DD1)) by the full-swing amplifier circuit 4903 attime F.

The signal amplified to the power supply voltage is read to the OUT nodeby switching on the transfer gate consisting of MTG3A and the like.

The process up to this point is the operation of a single cycle, and theoperation is returned to pre-charging the bit lines when reading orwriting is to be carried out again.

Described herein is the operation of reading data to the OUT node, butthe memory cells 5303 and 5304 are also simultaneously refreshed at thistime. In other words, when the full-swing amplifier circuit 4903 isactivated by SE1 and SE2 at time F, a high level is applied to the wordline (WL-EVN, in this case). Therefore, the signal of the bit lineamplified to the power supply voltage is written unchanged to the memorycell (5304, in this case), and the data of the memory cell is refreshed.

Described next is the operation of writing 0V from the data bus to thecapacitance C1 in the memory cell 5304.

The time from A to F and the driving by which the body-potential resetpulse is applied to the low-amplitude preamp circuit 4902 are the sameas in the above-described operation for the case in which data is readfrom the memory cells 5303 and 5304 to the OUT node. Therefore, theprocess that follows time F is described here.

MTG1A is switched on at time G. The transfer gate consisting of MDRGTand the like is switched on by a column decoder (not shown) at thistime, and M13 is switched on by WL_EVN. Therefore, 0V that appears onthe data bus can be written in the capacitance C1 through the pass ofthe bit line EVN (5301 b) and the transistor M13. At this point, thefull-swing amplifier circuit 4903 is in a latched state, but theimpedance of the data bus, the transfer gate composed of MTG1A and thelike, and MDRGT is sufficiently low, and the latched state can beinverted to thereby write data.

The process up to this point is the operation of a single cycle, and theoperation is returned to pre-charging the bit lines when reading orwriting is to be carried out again.

The sensitivity of a latching sense amplifier circuit is increased byresetting the body potential, and a stable read operation can be carriedout without malfunction even if the absolute value of the ΔV is low. Forthis reason, the number of memory cells that can be connected to a pairof bit lines can be increased, and the memory capacity (the amount ofinformation that can be stored in memory, and not the capacitance valueof a memory cell) per unit of surface area can be increased.

After power is switched on, the operation of writing to the memory cellsis carried out prior to the operation of reading from the memory cells.A body-potential reset pulse is applied to the MOS transistor 4901 a and4901 b of the low-amplitude preamp circuit 4902 during the writeoperation, and malfunctions in a latching sense amplifier can thereforebe avoided even it is the first read operation after power is switchedon.

Since the sense amplifier has a two-stage configuration, the sensitivityof the entire latching sense amplifier circuit is improved, and stableread operations can be carried out even when the absolute value of theΔV is low. For this reason, the number of memory cells that can beconnected to a pair of bit lines can be increased, and the memorycapacity per unit of surface area can be increased as well.

Furthermore, since parasitic capacitance is not present between thesecond carrier substrate 2 and the capacitances thereof, the parasiticcapacitance that accompanies the capacitance of the DRAM portion(data-holding circuit 3) can be considerably reduced. As a result, thesize of the capacitance within the memory cell can be reduced and thenumber of memory cells that can be connected to a pair of bit lines canbe increased. Hence, the memory capacity per unit of surface area can beincreased.

In the examples described above, a DRAM is used as the data-holdingcircuit 3 in order to facilitate the description of the presentinvention, but the data-holding circuit may be one that uses anothertype of capacitance. Also, the structure of the capacitance may be onethat has two electrodes in the direction vertical to the substratesurface, or may be one that has two electrodes in the directionhorizontal to the substrate surface.

1. A display device comprising: a data-holding circuit having acapacitance; a display portion having a plurality of pixel electrodes; afirst carrier substrate on which the data-holding circuit and thedisplay portion are formed; and a second carrier substrate that isdisposed opposite to the first carrier substrate being placed above thedisplay portion, an opposing substrate being absent above an area inwhich the data-holding circuit is disposed, wherein a provider of thecapacitance comprises a sense amplifier circuit for amplifying andlatching a magnitude of an electric potential between two nodes; thesense amplifier circuit has a first and second latch circuit; thedisplay device has a transmission control portion being provided thatallows or disallows signal transmission between one of the two nodes andat least one of the first and second latch circuits; and the outputvoltage amplitude of the first latch circuit is less than the outputvoltage amplitude of the second latch circuit.
 2. The display deviceaccording to claim 1, further comprising a storage capacitor and apixel, wherein a dielectric body of the data-holding circuit whichconstitutes the capacitance in the data-holding circuit and a dielectricbody of the storage capacitor provided in the pixel are formed with thesame film.
 3. The display device according to claim 1, comprising: adrive circuit for driving the display portion being disposed on thefirst carrier substrate on which the data-holding circuit and thedisplay portion are disposed.
 4. The display device according to claim3, further comprising a storage capacitor, a pixel and a transistorconstituting the drive circuit, wherein a dielectric body of thedata-holding circuit which constitutes the capacitance within thedata-holding circuit and a dielectric body of the storage capacitorprovided in the pixel are formed with the same film as at least one ofgate oxide films of the transistor constituting the drive circuit.
 5. Adisplay device comprising: a data-holding circuit having a capacitance;a display portion having a plurality of pixel electrodes; a firstcarrier substrate on which the data-holding circuit and the displayportion are formed; and a second carrier substrate that is disposedopposite to the first carrier substrate being placed above the displayportion, an opposing substrate being absent above an area in which thedata-holding circuit is disposed wherein the data-holding circuitcomprises a DRAM that can refresh memory cells solely in a readoperation.
 6. The display device according to claim 5, wherein aretention time of the memory cells in the DRAM is longer than arepetition time of a read operation.
 7. The display device according toclaim 6, wherein a source and drain of at least one transistor in thedata-holding circuit has an LDD structure.